The present invention relates to substrates for high dielectric constant capacitors and more particularly powder substrates based on tantalum and/or niobium fabricated into porous masses that are electrolytically xe2x80x9cformedxe2x80x9d to establish a thin oxide of tantalum and/or niobium (normally tantalum and/or niobium pentoxide) as the dielectric layer. These are utilized with well known solid or wet electrolyte systems.
The tantalum/niobium powder substrates (primarily tantalum) have been utilized for over half a Century as materials of choice for highest capacitance, compact capacitors with low leakage, low electrical series resistance and high voltage breakdown levels, standing up well to demanding usage and quality control life tests of military, computer and telecommunications markets.
The state of the art capacitance level for electrolytic capacitors has moved up in the last decade from under 10,000 micro-farad volts per gram to over 50,000 through shrinkage of powder substrate size (with greater surface area of formed oxide in relation to weight and volume of the anodes, anode porosity control for greater effective access to the expanded area, sinter controls, doping of the substrate with phosphorous and in some instances nitrogen, silicon, or sulfur. Improvements in lead wire production, lead wire to anode bonding, forming routines, electrolytic systems and packaging have also been made.
However, these advanced high capacitance systems have produced new expectations as to leakage, series resistance, bias dependence, thermal stability generally, in capacitor production and usage, frequency stability, voltage breakdown and overall stability that have not been met or are only met with high yield losses. Nitrided Ta, Nb and other forms of Ta, Nb modification have helped with stability as well as capacitance but insufficiently in relation to expectations.
It is a principal object of the invention to provide a capacitor substrate system affording improved leakage, series resistance, bias dependence, thermal stability generally in capacitor production and usage, frequency stability, increased porosity leading to lower equivalent series resistance (xe2x80x9cESRxe2x80x9d) and low dissipation factor (xe2x80x9cDFxe2x80x9d), in relation to high CV/gram systems (30,000 and higher).
It is a related object to achieve such stability reliably and in high yields.
The objects of the invention are met through new tantalum-silicon and niobium-silicon systems preferably formed as mixtures of 90-98 wgt-% Ta, Nb and 2-10 wgt-% of Si powders mixed together. One can also add Si to a reactor for Na reduction of K2Tap7. One can also use Si based wetting agents in suspensions of Ta as a means of introducing Si to Ta in appropriate amounts and forms.
Enhancement (lowering) of bias dependence after heat treatment has been achieved and can be achieved reliably through the Taxe2x80x94Si substrate system and such result is now reasonably projected for similar Ta/Nbxe2x80x94Si substrate systems. Electrolytic porous anode capacitors made with such systems can afford stable performance at high voltage formations, and under conditions of high frequency usage.
The benefits of the present invention can also be realized in Ta/Nb-nitride systems and in systems of Si with Ta/Nb, Ta/Nb-nitride doped with known capacitance enhancing impurities such as P, Si, S.
The benefits of silicon addition include pore size control of sintered anodes and optimized porosity with generally larger pores and greater uniformity of pore size to enable a more certain effective electrolyte precursor access, effective electrolyte conduction paths and less degradation of capacitor performance associated with varying porosity.
One method to distribute Si homogeneously throughout produced Ta or Nb is by use of liquid organo-silicon compounds. Due to the desire for reduced oxygen and carbon content, the preferred organo-silicon compound would be in the silicone family. These compounds which are primarily made up of SiOH bonds will decompose during the high temperature treatment of the powders to Si in a reducing atmosphere.
The reducing atmosphere may be provided in the standard technology of the field but it is preferred to be Mg or H2, or NH to minimize contamination.
Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings in which: